A known complex intermediate frequency (IF) based receiver architecture is depicted in FIG. 1. The receiver in FIG. 1 is designed to process RF signals carried by two carriers simultaneously. The received radio frequency (RF) signals are first fed to a low-noise amplifier (LNA). The LNA is followed by quadrature RF down-converting mixers where the local oscillator (LO) frequency is set to the centre of the two carriers such that the two carriers will be placed on the same IF frequency. The complex output of the RF down-converting mixers represents both the carriers where the lower frequency carrier appears on the negative side and the higher frequency carrier appears on the positive side. IF filters (IFF) may be used to filter out these carriers while suppressing the interfering signals. A complex IF mixer is used to down-convert the two carriers to baseband after which channel select filtering (CSF) and analogue-to-digital conversion (ADC) is performed. The receiver, shown in FIG. 1, is simplified and is for the purpose of illustration only.
A further simplification can be made where only the frequency translation operations are considered. This is shown in FIG. 2 where the frequency translation for the lower side (LS) carrier is shown together with the phase relations for the LO signal components. The corresponding diagram for two carriers is shown in FIG. 3.
It is known that quadrature mixers suffer from gain and phase imbalance and that the performance in this respect is usually measured as image-rejection ratio (IRR). For a RF quadrature mixer the IRR is typically in the range of 30 to 40 dB. In the architecture described above, a finite image rejection will lead to that the lower side carrier effectively will leak into the upper side carrier and vice versa. Gain and phase imbalance will also be present in the IF filters and the complex mixers. The complex mixer will, however, have a much better IRR compared with the RF mixer as it operates with much lower frequencies.
Another scenario where even higher IRR might be required is when the first carrier has a first bandwidth and the second carrier has a second bandwidth, where the first bandwidth is narrower than the second bandwidth. The image frequency range for the second carrier will then be larger than the first carrier. This means that not only the first carrier may leak into the second carrier due to limited IRR but also other carriers present adjacent to the first carrier and within the image frequency range of the second carrier. The power of these signals may be much larger and therefore have a large impact on performance when interfering with the second carrier. This can only be mitigated by providing an improved IRR.